Retirement and Postretirement Employee Benefit Plans - Schedule of Net Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Employee Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other non-operating income (expense)	Other non-operating income (expense)	Other non-operating income (expense)
Components of net periodic postretirement cost:
Service cost	$ 10,805	$ 16,165	$ 17,369
Interest cost	24,924	17,606	16,883
Expected return on assets	(29,113)	(29,531)	(27,913)
Amortization of prior service (credit) cost	(121)	(231)	(231)
Recognized actuarial (gain) loss	0	4,638	8,686
Net periodic pension cost	$ 6,495	$ 8,647	$ 14,794
Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other non-operating income (expense)	Other non-operating income (expense)	Other non-operating income (expense)
Components of net periodic postretirement cost:
Service cost	$ 845	$ 1,129	$ 1,067
Interest cost	4,227	2,647	3,180
Recognized actuarial (gain) loss	691	3,166	3,560
Settlements	1,030	0	9,151
Net periodic pension cost	$ 6,793	$ 6,942	$ 16,958
Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other non-operating income (expense)	Other non-operating income (expense)	Other non-operating income (expense)
Components of net periodic postretirement cost:
Service cost	$ 6,183	$ 10,235	$ 11,000
Interest cost	13,911	10,734	15,372
Expected return on assets	(11,215)	(13,249)	(10,455)
Amortization of prior service (credit) cost	(13,142)	(13,234)	30,533
Recognized actuarial (gain) loss	(7,452)	0	1,172
Net periodic pension cost	$ (11,715)	$ (5,514)	$ 47,622